Boxing Day Kaka Count– and some conclusionsHow many pesky parrots in
pohutukawas, plums or pines?by John Ogden

Kaka is an endemic species, found nowhere
else but New Zealand. Nationally kaka numbers have declined dramatically
since European arrival, and continued to do so between nation-wide
surveys in 1985 and 2004 (Robertson et al. 2007). Stoat- and Possum-free
Great Barrier Island is recognized as one of the major kaka population
centres, so it is important to get to know how many are here. There is
also local concern about kaka numbers as they deplete yield on fruit
trees and make orchards uneconomic.

A total of 5 kaka counts have been made on
Great Barrier since the first boxing-day count in December 2007. After
that, the next three counts were all made in the winter period, between
2008 and 2010. As the original boxing day count indicated much larger
numbers of kaka than the subsequent winter counts, it was thought
prudent to do another summer count to check that the seasonal
differences indicated were real, and not just a consequence of huge
‘counter enthusiasm’ on the first count for instance! The ‘final’ count
was on boxing-day 2010, and here I summarise the results for that count,
and make comparisons with the earlier data.

First, it is very pleasing to report on
the level of support shown to the Trust by participation in these
counts. While I’m not so naive as to think that all participants support
all the GBICT aspirations, presumably participation does indicate an
interest in understanding more about this endangered bird. Ninety-one
participants sent in 193 data sheets over the five counts, with 30
people participating in up to three counts (Fig 1). Participation per
count ranged from 33 to 45 persons. Actually participation is greater
than this, because some ‘participants’ were more than one person!
Anyway, you all know who you are, so a big ‘thankyou’ from the Trust!

The Boxing-day 2010 results are in Table
1, which follows the format used in previous reports (GBI Environmental
News #13, 16, 23). Individual sheets were grouped by ‘location’ (e.g.
there were three sheets from Awana). The Total AM and PM are the sums of
all counts by all observers and this assumes that all birds are
different individuals. ‘Minimum at location’ takes the largest number
actually counted by any observer at the location as the minimum that
must have been present (i.e. there were at least that number). Maximum
uses the largest ‘numbers usually seen over past two weeks’ or the
largest count at the location on the day if that was greater.

Some quite large flocks were recorded in
2010, for example 39 birds at Tryphena and 26 at Whangapara-para. It is
very difficult to count the birds in fast-moving groups flying or
feeding, and it is likely that some birds were counted twice. However,
in these two cases (and others), large numbers were also recorded by
other observers at the location, so they are considered to be reliable.

All five data sets are compared in Table 2
and Fig 2. These results clearly demonstrate that numbers decline in
winter, which coincides with an increase in numbers on the mainland
(personal communications: Suzi Phillips; see www.kakawatchnz.org ). It
is not known how the data in Table 2 relate to the true population size
of kaka on the Island, as clearly not all individuals are counted on
each occasion, and there may be double counting in some cases. “Maximum
based on locations” is probably the best estimate, but this may
underestimate numbers, especially in summer when the birds are more
dispersed through remote bush areas for breeding. Overall it appears
that the summer breeding population is between 200 – 300 birds (guess at
probably < 100 breeding pairs), while the overwintering population is
probably c. 125 birds ± 50.

Notes: nc = not counted. (1) This minimum
is the maximum actually counted by one of the observers at a ‘location’
(i.e. at least this number was definitely present at the location). (2)
This maximum is based on either the maximum actually counted by an
observer at the location or (usually) the estimate of numbers usually
seen by the observer over the last two weeks.

The data (Fig 2) might suggest that kaka numbers have increased in
summer between 2007 and 2010. However, there is no proof that this is so
because the error bars overlap. Moreover, one important location (Motairehe)
wasn’t counted in 2007. This could easily account for the difference. It
is also very difficult to separate real population changes from changes
in conspicuousness in this species.

Movement from Great Barrier to the
mainland appears to be in April and May, while the return is in
September. Our previous (2006-08) Island-wide bird counts clearly show
that by September kaka are as conspicuous in most places as they are in
December (Table 3). Those results also suggest that the summer
population is about twice the winter, though numbers remain in some
locations where food is available.

Not everyone noted what tree species the
kaka frequented; often they were only seen flying. However, the data set
has 196 records of tree species visited by kakas, with roughly as many
winter as summer records. If we express the records for each species as
a percentage of all records (Table 4), and assume that presence in a
tree is proportional to the value of the tree as a food source (an
assumption that needs considerable qualification!), then we can make a
few conclusions.

Native plants are most important in
December. Pohutukawa (presumably mostly flowering) is top of the list,
comprising 30% of all observations. Puriri and flax are also important.
Fruit trees comprise 17% of observations, though of course many do not
have ripe fruit in December. In the Winter native trees seem to be less
important, but puriri (which carries some fruit almost throughout the
year), is significant. Exotic trees, especially pines, gum trees and
Banksia, seem to be important for the (smaller) overwintering
population. Citrus are important in the orchard fruits category, which
overall stays about the same as in summer (17 vs. 19%).

It would be unwise to make too much of
these data, but they do seem to confirm the impressions gained from
other observations; just as abundant food supply may trigger kaka
breeding (Powlesland et al. 2009), food shortage may be what drives over
half the birds off-island in the winter. At that time it is the exotic
trees, especially pines, which comprise the main food source.

Exotic trees, especially pines, are seen
mainly as problem ‘weeds’ on Great Barrier by the Department of
Conservation and some land-owners. These data add another perspectives.
Pines comprise an important food resource for kaka (perhaps more than
18% of calories in winter?). If exotics such as pines, macrocarpa, gums,
Banksia, wattles and coral trees were totally removed from the Island,
kaka would either leave for the mainland in winter – where they might be
at much greater risk – or else seek other food sources. It seems most
likely that they would turn to “fruit trees” rather than the native
forest, which has so little food that half the population already
leaves.

All we can really conclude from Table 4 is
that native trees (especially Pohutukawa) are important to kaka in
December, and exotics (especially pines) are important in winter. The
data tell us nothing about de-predations on orchards, or the economic
effects, although they indicate a problem for anyone trying to grow
fruit commercially – we already knew that! The point is that having
numbers shifts the argument to a new level – for example the data
support the view that pines are probably the most important food source
for kaka in the winter. This knowledge should be used to formulate new
ecosystem management practices to balance the kaka population numbers at
a sustainable level - which also allows humans to grow fruit. Currently
kaka are the meat in the sandwich in a conflict between conservation
aims, and the legitimate aspirations of many fruit tree owners on Great
Barrier.

Fig 2. Mean, maxima and minima
estimates of kaka counts based on locations. Winter counts in white,
summer counts in grey. The bars give the average, the thin lines give
the maxima and minima(“Error bars”).

Table 3. Probability (% frequency) of
seeing or hearing a kaka (at any distance) in a five minute stop in
different locations on Great Barrier. Data from GBICT bird counts
2006-2008 and Glenfern Sanctuary 2002–2006.

Notes: nc = not counted.
(1) This minimum is the maximum actually counted by one of the observers
at a ‘location’ (i.e. at least this number was definitely present at the
location).
(2) This maximum is based on either the maximum actually counted by an
observer at the location or (usually) the estimate of numbers usually
seen by the observer over the last two weeks.

Table 4. Relative frequencies of records
of trees visited by kaka (from all five data sets).